In-Situ Monitoring of Alloy Dissolution and Residual Film Formation during the Pretreatment of Al-Alloy AA2024-T3

Abstract

The dissolution of intermetallic phases in AA 2024-T3 aluminum alloy sheet was investigated during a coating pretreatment sequence. The atomic emission spectroelectrochemistry (AESEC) technique was used to quantitatively measure the dissolution rates of individual alloying elements during a complete pretreatment sequence. The results demonstrate the significant selective dissolution of Al in 1.25 M NaOH, leading to the enrichment of alloying elements such as Cu. Subsequent 2.8 M HNO 3 treatment contributes toward dissolving the excess residual layer and passivates the alloy matrix, however the presence of a residual Cu layer at the alloy surface was evidenced. The real-time alloy dissolution profiles are presented herein, and discussed in the context of the surface morphology via microscopy. The surface treatment of aluminum (Al) alloys is required prior to the application of corrosion protective coatings.1 Protective coatings are essential in order to allow microstructurally complex alloys, which are nominally prone to localized corrosion, 2-7 to be used in service; particularly in aerospace applications.8 Coatings for Al-alloys are nominally multilayered coating 'systems', with the first coating being a chemical conversion coating. The surface treatment prior to this chemical conversion coating is as important as the coating itself, since it determines the efficacy of any conversion coating.1,9 Such surface treatment, often termed pre-treatment, generally involves alkaline etching and acid pickling. Sodium hydroxide (NaOH) is often used to remove any organic residue and also to remove several micrometers of the alloy surface.10 This is followed by acid pickling, usually performed in nitric acid (HNO 3 ) to remove any residue, products, films, or particles remnant from -or caused by -the prior alkaline step. The purpose of surface pretreatment is to provide a chemically homogeneous surface chemistry prior to subsequent coating processes. The chemical heterogeneity of the Al-alloy surfaces is due to the many alloying elements present, such as copper (Cu), magnesium (Mg), iron (Fe) and manganese (Mn). Cu improves the mechanical properties of AA 2024-T3 11 however decreases corrosion resistance due to stimulating various phases which serve as local electrochemical entities, in addition to raising the alloy potential to above the pitting potential of the matrix phase 7,[12][13][14][15][16] . This element is considered to be a major contributor to the localized corrosion of Al 2,17,18 and its elimination is one of primary goals of aluminum alloy surface pretreatment.Knowledge of the elementary dissolution kinetics during the pretreatment sequence would be very useful for the development of surface treatment formulations, however, this information is often very difficult to obtain. In situ monitoring of the corrosion reactions during surface treatment is difficult as the corrosion reactions occur within a relatively short time with large reaction rates and may involve extensive precipitation of dissolution products a...

Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation

“…A simulation of a complete pretreatment sequence is shown in Figure 11, in this case involving NaOH/water rinse/HNO 3 under the conditions indicated in the figure. 69 The results demonstrate the intense Al dissolution in alkaline solution, followed by the dissolution of residual Cu and the passivation of the Al in the nitric acid step. The quantitative aspects of AESEC are well illustrated: Al dissolution was rapid in the alkaline solution corresponding to an etching rate of approximately 160 μg/h.…”

“…The red dashed curve shows the RTD for the flow cell under the conditions of these experiments. Modified from Gharbi, et al,69 under CC-BY-NC-ND license.…”

Self-Healing Effect of Hybrid Sol-Gel Coatings Based on GPTMS, TEOS, SiO2Nanoparticles and Ce(NO3)3Applied on Aluminum Alloy 7075-T6

“…It is known that the deposition of Ce-oxides proceeds at the cathodic sites at the surface which are, in the case of AA7075-T6, Cu-containing intermetallic particles (IMPs). [54][55][56][57][58][59] The fact that no Cu could be identified at the intermetallic particles proves that the deposits of Ce hydroxide/oxide are thicker than is the EDS analysis depth, i.e. several micrometers.…”

Solute Capture and Doping of Al in Cu2O: Corrosion, Tarnish Resistance, and Cation Release of High-Purity Cu-Al Alloys in Artificial Perspiration

“…A cuprous (n = 1) assumption provided the best match between mass loss and derived anodic charge obtained independently from EIS measurements and corroborated by prior work. 30,38,44,45 Electrochemical impedance measurements on CuAl samples exposed to artificial perspiration also revealed Cu-2Al had the greatest corrosion rate (1/R p α i corr , Figure 12a). Cu and the other Cu-Al alloys had similar mass loss at 96 h. affect anodic kinetics.…”